If and when you make coffee, bring me some. (Your making coffee is not a given, but should you do so, bring me some at the time when you do make it.)
There’s no doubt, though, that the real value of the expression is not its logical quality but its emphatic quality and the implications it carries. It doubly specifies, and thus has the insistence and intensity of reiteration. It means there is some doubt as to the eventuality, and perhaps some impatience regarding it.